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Date:   Fri, 22 Jan 2021 13:26:57 +0000
From:   Will Deacon <will@kernel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        iommu <iommu@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Yang Yingliang <yangyingliang@huawei.com>
Subject: Re: [PATCH 1/1] iommu/arm-smmu-v3: add support for BBML
Message-ID: <20210122132657.GF24102@willie-the-truck>
References: <20201126034230.777-1-thunder.leizhen@huawei.com>
 <20210122125132.GB24102@willie-the-truck>
 <34a9c164-389d-30cd-11a3-8796eb7bca93@arm.com>
 <ad8cb736-6760-8aab-b11e-f296ec12301b@huawei.com>
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On Fri, Jan 22, 2021 at 09:21:20PM +0800, Leizhen (ThunderTown) wrote:
> On 2021/1/22 21:00, Robin Murphy wrote:
> > On 2021-01-22 12:51, Will Deacon wrote:
> >> On Thu, Nov 26, 2020 at 11:42:30AM +0800, Zhen Lei wrote:
> >>> ? -??????? tblp = ptep - ARM_LPAE_LVL_IDX(iova, lvl, data);
> >>> -??????? if (__arm_lpae_unmap(data, NULL, iova, sz, lvl, tblp) != sz) {
> >>> -??????????? WARN_ON(1);
> >>> -??????????? return -EINVAL;
> >>> +??????? switch (cfg->bbml) {
> >>> +??????? case 0:
> >>> +??????????? /*
> >>> +???????????? * We need to unmap and free the old table before
> >>> +???????????? * overwriting it with a block entry.
> >>> +???????????? */
> >>> +??????????? tblp = ptep - ARM_LPAE_LVL_IDX(iova, lvl, data);
> >>> +??????????? if (__arm_lpae_unmap(data, NULL, iova, sz, lvl, tblp) != sz) {
> >>> +??????????????? WARN_ON(1);
> >>> +??????????????? return -EINVAL;
> >>> +??????????? }
> >>> +??????????? break;
> >>> +??????? case 1:
> >>> +??????????? __arm_lpae_init_pte(data, paddr, prot, lvl, ptep, ARM_LPAE_PTE_nT);
> >>> +
> >>> +??????????? io_pgtable_tlb_flush_walk(iop, iova, sz, ARM_LPAE_GRANULE(data));
> >>> +??????????? tblp = iopte_deref(pte, data);
> >>> +??????????? __arm_lpae_free_pgtable(data, lvl + 1, tblp);
> >>> +??????????? break;
> >>> +??????? case 2:
> >>> +??????????? __arm_lpae_init_pte(data, paddr, prot, lvl, ptep, 0);
> >>> +
> >>> +??????????? io_pgtable_tlb_flush_walk(iop, iova, sz, ARM_LPAE_GRANULE(data));
> >>> +??????????? tblp = iopte_deref(pte, data);
> >>> +??????????? __arm_lpae_free_pgtable(data, lvl + 1, tblp);
> >>> +??????????? return 0;
> >>
> >> Sorry, but I really don't understand what you're trying to do here. The old
> >> code uses BBM for the table -> block path so we don't need anything extra
> >> here. The dodgy case is when we unmap part of a block, and end up installing
> >> a table via arm_lpae_split_blk_unmap(). We can't use BBM there because there
> >> could be ongoing DMA to parts of the block mapping that we want to remain in
> >> place.
> >>
> >> Are you seeing a problem in practice?
> > 
> > Right, I was under the assumption that we could ignore BBML because we
> > should never have a legitimate reason to split blocks. I'm certainly not
> > keen on piling any more complexity into split_blk_unmap, because the
> > IOMMU API clearly doesn't have a well-defined behaviour for that case
> > anyway - some other drivers will just unmap the entire block, and IIRC
> > there was a hint somewhere in VFIO that it might actually expect that
> > behaviour.
> 
> I'm going home. I'll answer you two tomorrow.

It can wait until Monday! Have a good weekend :)

Will